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Image Viewer Rendering

The ImageViewer rendering cycle supports full customization. The various events and their data allow for any of the following:
  • Add user-defined rendering code on the view or the items. For example, the LEADTOOLS annotations framework hooks to the rendering cycle to draw the annotations container on top of an item.

  • Perform full owner drawing of any part of the view or item.

  • Re-direct the rendering to an external context. For example, the ImageViewerPanControl uses redirection to duplicate the current appearance of the viewer to an external control.

Rendering Cycle

The rendering cycle occurs when ImageViewer.Invalidate or ImageViewer.RenderRedirect is called due to normal control painting or a response to ImageViewer.UpdateTransform. To customize the rendering cycle, you can either subscribe to the specific event or derive from the ImageViewer and override the specific virtual method. All these methods and event use the ImageViewerRenderEventArgs class to hold the information needed.

ImageViewerRenderEventArgs contains the following:

Member Description

In JavaScript, the CanvasContext object of the foreground canvas of the viewer - ImageViewer.ForeCanvas) or that of the target device during RenderRedirect.


The item being rendered or null of this part of the rendering cycle is view-specific.


The item part being rendered or View if this part of the rendering cycle is is view-specific.


An error that was caught during the rendering cycle. Used with the ImageViewer.RenderError event.


Current clipping rectangle to be applied.

The image viewer will call the virtual methods and fires the events in the order described below. The ImageViewerRenderEventArgs.Contex property will point to the System.Drawing.Graphics (in Windows.Forms) or (HTML CanvasContext2D object for the fore canvas in JavaScript) the viewer control or the redirect target.

Note: If the image view is using Elements Mode, then not all of these events will occur.

Method - Event ImageViewerRenderEventArgs.Item, Part Default Operations
OnEraseBackground - EraseBackground

null - ImageViewerItem.View

Erases or clears the background (Not in Elements Mode)
OnPreRender - PreRender

null - ImageViewerItem.View

Nothing. Can be used to set the context properties (for example, anti-aliasing or interpolation quality)
OnRenderShadow - RenderShadow

null - ImageViewerItem.View

Renders the drop shadow of the view (Not in Elements Mode)
OnRenderBorder - RenderBorder

null - ImageViewerItem.View

Renders the border of the view (Not in Elements Mode)
For each item

Loops through the items in the viewer. Only occur if the item is IsVisible, has none empty ImageSize and if it intersects with the current visible viewer surface and clipping rectangle.

OnPreRenderItem - PreRenderItem

The item - Item

OnRenderItem - RenderItem

The item - Item

OnRenderBackground - RenderBackground

The item - Item

Fills the content of the item with the background color based on the item state (Not in Elements Mode)
OnRenderBorder - RenderBorder

The item - Content

Renders the content border using the border color based on the item state (Not in Elements Mode)
OnRenderShadow - RenderShadow

The item - Image

Renders the drop shadow around the image of the item (Not in Elements Mode)
OnRenderImage - RenderImage

The item - Image

Renders the item image (Not in Elements Mode)
OnRenderBorder - RenderBorder

The item - Image

Renders the border of the image (Not in Elements Mode)
OnRenderText - RenderText

The item - Text

Renders the text of the item (Not in Elements Mode)
OnPostRenderItem - PostRenderItem

The item - Item

Next item
OnPostRender - PostRender

null - ImageViewerItem.View

Renders the floaters and regions
OnRedirectRender - RedirectRender

null - ImageViewerItem.View


The most common custom rendering is performed by subscribing to these events:

  • PostRenderItem: To render extra information on top of an item

  • PostRender: To render extra information on top of the whole viewer

Refer to these events for an example.

To manually trigger a new rendering cycle, call Invalidate to invalidate a portion of the whole surface of the viewer. This method accepts a rectangle that specifies the portion of the viewer to update for optimal speed. The InvalidateItem or InvalidateItemByIndex methods can be used to re-render a single item quickly.

Errors when Rendering

Rendering of the ImageViewer control normally occurs automatically and not through any code called by the user. If an error occur, the user does not have the ability to try/catch the exception and process the error. Therefore, ImageViewer contains the RenderError events. This will occur when an error happens in the internal rendering cycle. The event uses the same ImageViewerRenderEventArgs data class, so all the usual parameters of what item and part caused the error are passed to the user. In addition, it will catch the error and set it in the ImageViewerRenderEventArgs.Error property.

Other Rendering Operations

The ImageViewer also supports the following operations related to rendering:


Member Description

Display all items with inverted colors


Update the value of the screen resolution in dots per inch


Account for the resolution of the image in the items when calculating the display size


Manually change the aspect ratio used to compensate for non-square pixels


Temporally disable rendering


Re-enable rendering


Request that the viewer re-renders the whole or part of the view


Render the current view to an external context


Display only the part of the image inside its content if the item transformation result in a larger boundary


Turn off or on all automatic rendering of view and items

Help Version 23.0.2024.5.10
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Products | Support | Contact Us | Intellectual Property Notices
© 1991-2023 LEAD Technologies, Inc. All Rights Reserved.